Magnetoresistive random access memory (MRAM) utilizes magnetic tunnel junctions (MTJs) to store digital information. A MTJ typically comprises a pinned magnetic layer and a free magnetic layer separated by a dielectric barrier layer. The pinned magnetic layer has a magnetic orientation which is fixed in a preferred direction while the free magnetic layer is allowed to switch direction when exposed to an applied magnetic field. The resistance of the device depends on the magnetic orientation of the layers on either side of the barrier layer. If the magnetic orientations of the layers are parallel relative to one another, the resistance across the barrier is lower, while if the magnetic orientation of the layers are antiparallel relative to one another, the resistance is higher. The relative change in resistance is termed magnetoresistance, and is expressed in a percentage change with respect to the lower resistance value.
In order to switch a MTJ (i.e. write the memory cell), magnetic fields are applied using on-chip currents through wordlines and bitlines placed near the device. One MTJ architecture relies on the switching of a free layer composed of a single material that is patterned into a sub-micron island. In this configuration, the single-component free layer is switched by the application of fields in the two in-plane directions by currents flowing through the wordlines and bitlines. Half-selected bits (i.e. those which are subject to fields in only one of the two in-plane directions) are not switched, due to the asteroid-shaped switching curve that characterizes the magnetic switching behavior of these devices. Nevertheless, while such an architecture may work reliably for larger MTJs, it becomes problematic as the device geometries shrink. As the lateral dimensions of the single-component free layer are reduced, the activation energy of the MTJ against thermally activated switching is also reduced, even for half-selected devices. This can make it difficult to switch one selected MTJ in an array without also inadvertently switching other devices.
Recently, a MTJ architecture with a free layer comprising two or more magnetic sublayers has been developed that avoids these problems, while also providing other advantages. Where two magnetic sublayers are utilized in the free layer, for example, these free magnetic sublayers are generally antiparallel to one another and separated by a thin non-magnetic layer. With this configuration, the multicomponent free layer is switched by applying current to the wordlines and bitlines in a sequence of timed pulses so as to induce a direct-write or toggle-write phenomenon in the free magnetic sublayers. This multicomponent free layer configuration has the advantage of providing a highly selective method of writing to the MTJs in an array due, at least in part, to an enhanced switching activation energy of half-selected devices.
Despite these advantages, however, MTJs with free layers comprising two or more magnetic sublayers are susceptible to geometric effects that are not present in MTJs with single-component free layers. More specifically, because one of the magnetic sublayers in a multicomponent free layer device is slightly closer to the pinned layer, it may feel a stronger magnetic field from the pinned layer than the other free magnetic sublayers. This field difference, averaged over the device area, can be many tens of Oersted and becomes more pronounced as the device becomes smaller. Such an asymmetry in magnetic fields acting on the free magnetic sublayers adversely affects the switching of the device. Therefore, only when this asymmetry is reduced will the largest toggle window (i.e., the greatest range of magnetic fields over which toggle-write operations work) be obtained.
Consequently, there is a need to reduce the magnetic asymmetry present in MTJs with free layers comprising multiple magnetic sublayers.